A one-dimensional array or a two-dimensional array of imaging sensors may be connected to processing circuitry in order to generate image information relating to an area at which the array is directed. Each sensor generates a signal that is responsive to the light received at the sensor. The signal may represent intensities of colors of light received by the sensor.
Charged coupled devices (CCDs) are used in many imaging applications. An element of a CCD array integrates a charge that is generated by incoming light, with the charge being accumulated in a potential well along the surface of a semiconductor chip on which the CCD array is fabricated. The accumulated charges of the array are sequentially shifted to on-chip circuitry that transfers the signals to other circuitry that is not integrated onto the chip.
While a CCD operates well for its intended purposes, the “gain” that is experienced at each element (i.e., pixel) is less than one. The gain may be measured at the current level (output current/photon current) or at the electron level (output electrons/input photons). With a gain at the electron level that has a maximum of unity, the signal-to-noise ratio at the output of a particular element is limited.
U.S. Pat. No. 6,018,187 to Theil et al. describes the use of PIN diodes as an alternative to CCD technology. A pixel interconnect structure is formed on the top surface of a substrate, such as a silicon substrate. The pixel interconnect structure is a dielectric layer, such as silicon oxide or silicon nitride, having conductive vias extending from selected locations on the surface of the substrate to bottom electrodes of an array of photosensors. Preferably, the bottom electrodes are formed of amorphous silicon having an n-type dopant. A continuous intrinsic layer is formed over the bottom electrodes. A continuous layer of p-doped amorphous silicon covers the intrinsic layer, while a transparent conductor covers the p-doped layer. As a result, a PIN diode is formed for each one of the bottom electrodes. The PIN diodes are “elevated” sensors, since they are positioned above the surface of the substrate. The pixel interconnect structure allows the elevated PIN diodes to be electrically linked to processing circuitry that is fabricated at the substrate level.
An unrelated alternative to CCD technology is described in U.S. Pat. No. 5,763,909 to Mead et al. Bipolar phototransistors are fabricated by introducing dopants into a silicon substrate. Conventional dopant implantation steps are utilized. When an array of the bipolar phototransistors is formed, the bases of the phototransistors in a particular row are capacitively coupled to a common row-select line. The emitters of all the phototransistors in a particular column are connected to a common column-select line. Thus, each phototransistor can be individually selected. The array of phototransistors provides greater gain than is available using CCD technology.
An object of the invention is to provide an imaging arrangement in which individual photosensors in an array of photosensors achieve a relatively high level of gain, so that a desired signal-to-noise ratio can be obtained.